Structure and optical properties of upper In(Ga)As quantum dots on a different seed layer with a thin GaAs spacer layer

The structure and optical properties of In(Ga)As grown with the introduction of InGaAlAs or InAlAs seed dots layers are investigated. The area density and size homogeneity of the upper InGaAs dots are efficiently improved with the introduction of a layer of high-density buried dots. When the GaAs spacer layer is too thin to cover the seed dots, the upper dots exhibit the characterization of a quantum well. By analyzing the growth dynamics, we refer to it as an empty-core structure dot. (C) 2002 Elsevier Science B.V. All rights reserved.

Abnormal effect of growth interruption on GaSb quantum dots formation grown by molecular beam epitaxy

The growth interruption (GI) effect on GaSb quantum dot formation grown on GaAs by molecular beam epitaxy was investigated. The structure characterization was performed by reflection high-energy electron diffraction (RHEED), along with photoluminescence measurements. It is found that the GI can significantly change the surface morphology of GaSb QDs. During the GI, the QDs structures can be smoothed out and turned into a 2D-like structure. The time duration of the GI required for the 3D/2D transition depends on the growth time of the GaSb layer. It increases with the increase of the growth time. Our results are explained by a combined effect of the stress relaxation process and surface exchange reactions during the GI. (C) 2002 Elsevier Science B.V. All rights reserved.

Luminescence study of (11(2)over-bar0) GaN film grown by metalorganic chemical-vapor deposition

Investigations on photoluminescence properties of (11 (2) over bar0) GaN grown on (1 (1) over bar 02) Al2O3 substrate by metalorganic chemical-vapor deposition are reported. Several emission lines not reported before are observed at low temperature. The sharp peak at 3.359 eV is attributed to the exciton bound to the neutral acceptor. Another peak at 3.310 eV represents a free-to-bound, probably a free electron-to-acceptor, transition. The 3.241 and 3.170 eV lines are interpreted as phonon replica lines of the 3.310 eV line. The phonon energy is 70 meV, consistent with the energy of transverse optical E-1 phonon. The optical properties of the lines are analyzed. (C) 2003 American Institute of Physics.

Interface-related in-plane optical anisotropy in GaAs/AlxGa1-xAs single-quantum-well structures studied by reflectance difference spectroscopy

The in-plane optical anisotropies of a series of GaAs/AlxGa1-xAs single-quantum-well structures have been observed at room temperature by reflectance difference spectroscopy. The measured degree of polarization of the excitonic transitions is inversely proportional to the well width. Numerical calculations based on the envelope function approximation incorporating the effect of C-2v-interface symmetry have been performed to analyze the origin of the optical anisotropy. Good agreement with the experimental data is obtained when the optical anisotropy is attributed to anisotropic-interface structures. The fitted interface potential parameters are consistent with predicted values.

Type-II SiGe/Si MQWS (multi-quantum wells) and self-organized Ge/Si islands grown by UHV/CVD system

Type-II SiGe/Si MQWs (Multi-Quantum Wells) and Self-Organized Ge/Si Islands were successfully grown by a homemade ultra-high vacuum/chemical vapor deposition (UHV/CVD) system. Growth characteristics and PL (photoluminescence) spectra at different temperature were measured. It demonstrated that some accumulation of carriers in the islands results in the increase of the integrated PL intensity of island-related at a certain temperature range.

Nitrogen vacancy scattering in GaN grown by metal-organic vapor phase epitaxy

Electron mobility limited by nitrogen vacancy scattering was taken into account to evaluate the quality of n-type GaN grown by metal-organic vapor phase epitaxy. Two assumptions were made for this potential for the nitrogen vacancy (1) it acts in a short range, and (2) does not diverge at the vacancy core. According to the above assumptions, a general expression to describe the scattering potential U(r) = - U-0 exp[- (r/beta)(n)], (n = 1, 2,...,infinity) was constructed, where beta is the potential well width. The mobilities for n = 1, 2, and infinity were calculated based on this equation, corresponding to the simple exponential, Gaussian and square well scattering potentials, respectively. In the limiting case of kbeta << 1 (where k is the wave vector), all of the mobilities calculated for n = 1, 2, and infinity showed a same result but different prefactor. Such difference was discussed in terms of the potential tail and was found that all of the calculated mobilities have T-1/2 temperature and beta(-6) well width dependences. A mobility taking account of a spatially complicate scattering potential was studied and the same temperature dependence was also found. A best fit between the calculated results and experimental data was obtained by taking account of the nitrogen vacancy scattering. (C) 2002 Elsevier Science Ltd. All rights reserved.

Crystallographic tilt in GaN layers grown by epitaxial lateral overgrowth

The crystallographic tilt in GaN layers grown by epitaxial lateral overgrowth (ELO) on sapphire (0001) substrates was investigated by using double crystal X-ray diffraction (DC-XRD). It was found that ELO GaN stripes bent towards the SiNx mask in the direction perpendicular to seeding lines. Each side of GaN (0002) peak in DC-XRD rocking curves was a broad peak related with the crystallographic tilt. This broad peak split into two peaks (denoted as A and B), and peak B disappeared gradually when the mask began to be removed by selective etching. Only narrow peak A remained when the SiNx mask was removed completely. A model based on these results has been developed to show that there are two factors responsible for the crystallographic tilt: One is the non-uniformity elastic deformation caused by the interphase force between the ELO GaN layer and the SiNx mask. The other is the plastic deformation, which is attributed to the change of the threading dislocations (TDs)-from vertical in the window regions to the lateral in the regions over the mask.

Study of GaN thin films grown on vicinal SiC (0001) substrates by molecular beam epitaxy

A detailed study of the characteristics of undoped GaN films, grown on either vicinal or nominal flat SiC (0001) substrates by molecular beam epitaxy, has been carried out using photoluminescence and Raman scattering techniques. The I I K photoluminescence spectra of the GaN film grown on the vicinal SiC (0001) substrate show a strong and sharp near-bandgap peak (full width at half maximum (FWHM) similar to 16 meV). This feature contrasts with that of the GaN film grown on the nominal flat SiC (0001) substrate where the I I K photoluminescence spectra exhibit the near-bandgap peak (FWHM similar to 25 meV) and the intensity is approximately seven times weaker than that of the vicinal film sample. The redshift of the near-bandgap peak associated with excitons bound to shallow donors is related to the stress caused by both the lattice mismatch and the thermal expansion coefficient difference between GaN and SiC substrates. The measured thermal activation energy of the shallow donor of 33.4 meV is determined by using an Arrhenius plot of the near-bandgap luminescence versus I IT from the slope of the graph at high temperature. The temperature dependence of the FWHM of the near-bandgap luminescence has also been studied. The Raman scattering measurements from the vicinal film reveal that the E-2 phonon peak is strengthened and the A(1)(LO) phonon peak is shifted towards the low-frequency side with enhanced intensity, in comparison to that from the nominal flat film, suggesting a reduction in the density of defects and a lower free carrier concentration in the vicinal GaN film.

The effects of concomitant In and N incorporation on the photoluminescence of GaInNAs

We investigated the effects of concomitant In- and N-incorporation on the photoluminescence (PL) of GaInNAs grown by molecular beam epitaxy. In comparison with the N-free GaInAs epilayer, the PL spectra of the GaInNAs epilayer exhibit an anomalous S-shape temperature dependence of dominant luminescence peak. Through further careful inspection, two PL peaks are clearly discerned and are associated with the interband excitonic recombinations and excitons bound to N-induced isoelectronic impurity states, respectively. By comparing the PL spectra of GaInNAs/ GaAs quantum wells (QWs) with those of In-free GaNAs/GaAs QWs grown under similar conditions, it is found that the concomitant In- and N-incorporation reduces the density of impurities and has an effect to improve the intrinsic optical transition of GaInNAs, but also enhance the N-induced clustering effects. At last, we found that rapid thermal annealing can significantly reduce the density of N-induced impurities. (C) 2002 Elsevier Science B.V. All rights reserved.

Photovoltage and luminescence study of stacked InAs/GaAs self-organized quantum dots

The ground and excited state excitonic transitions of stacked InAs self-organized quantum dots (QDs) in a laser diode structure are studied. The interband absorption transitions of QDs are investigated by non-destructive PV spectra, indicating that the strongest absorption is related to the excited states with a high density and coincides with the photon energy of lasing emission. The temperature and excitation (electric injection) intensity dependences of photoluminescence and electroluminescence indicate the influence of state filling effect on the luminescence of threefold stacked QDs. The results indicate that different coupling channels exist between electronic states in both vertical and lateral directions.

Modulation spectroscopy of GaAs covered by InAs quantum dots

Contactless: electroreflectance has been employed at room temperature to study the Fermi level pinning at undoped-n(+) GaAs surfaces covered by 1.6 and 1.8 monolayer (ML) InAs quantum dots (QDs). It is shown that the 1.8 ML InAs QD moves the Fermi level at GaAs surface to the valence band maximum by about 70 meV compared to bare GaAs, whereas 1.6 ML InAs on GaAs does not modify the Fermi level, It is confirmed that the modification of the 1.8 ML InAs deposition on the Fermi level at GaAs surface is due to the QDs, which are surrounded by some oxidized InAs facets, rather than the wetting layer.

Effect of InAlAs/InGaAs cap layer on optical properties of self-assembled InAs/GaAs quantum dots

We have investigated the effect of InAlAs/InGaAs cap layer on the optical properties of self-assembled InAs/GaAs quantum dots (QDs). We find that the photoluminescence emission energy, linewidth and the energy separation between the ground and first excited states of InAs QDs depend on the In composition and the thickness of thin InAlAs cap layer. Furthermore, the large energy separation of 103 meV was obtained from InAs/GaAs QDs with emission at 1.35 pm at room temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

Photoluminescence study of self-assembled InAs/GaAs quantum dots covered by an InAlAs and InGaAs combination layer

We have fabricated a quantum dot (QD) structure for long-wavelength temperature-insensitive semiconductor laser by introducing a combined InAlAs and InGaAs overgrowth layer on InAs/GaAs QDs. We found that QDs formed on GaAs (100) substrate by InAs deposition followed by the InAlAs and InGaAs combination layer demonstrate two effects: one is the photoluminescence peak redshift towards 1.35 mum at room temperature, the other is that the energy separation between the ground and first excited states can be up to 103 meV. These results are attributed to the fact that InAs/GaAs intermixing caused by In segregation at substrate temperature of 520 degreesC can be considerably suppressed by the thin InAlAs layer and the strain in the quantum dots can be reduced by the combined InAlAs and InGaAs layer. (C) 2002 American Institute of Physics.

Structure and optical properties of self-assembled InAs/GaAs quantum dots with In0.15Ga0.85As underlying layer

A high density of 1.02 x 10(11) cm(-2) of InAs islands with In(0.15)Gao(0.85)As underlying layer has been achieved on GaAs (10 0) substrate by solid source molecular beam epitaxy. Atomic force microscopy and PL spectra show the size evolution of InAs islands. A 1.3 mum photoluminescence (PL) from InAs islands with In(0.15)Gao(0.85)As underlying layer and InGaAs strain-reduced layer has been obtained. Our results provide important information for optimizing the epitaxial structures of 1.3 mum wavelength quantum dots devices. (C) 2002 Elsevier Science B.V. All rights reserved.

Wafer bonding technique used for the integration of cubic GaN/GaAs (001) with Si substrate

We successfully used the metal mediated-wafer bonding technique in transferring the as-grown cubic GaN LED structure of Si substrate. The absorbing GaAs substrate was removed by using the chemical solutions of NH4OH : H2O2=1 : 10. SEM and PL results show that wafer bonding technique could transfer the cubic GaN epilayers uniformly to Si without affecting the physical and optical properties of epilayers. XRD result shows that there appeared new peaks related to AgGa2 and Ni4N diffraction, indicating that the metals used as adhesive and protective layers interacted with the p-GaN layer during the long annealing process. It is just the reaction that ensures the reliability of the integration of GaN with metal and minor contact resistance on the interface.